This application claims priority to Japanese Patent Application No. 2000-270722 filed on Sep. 6, 2000 in Japan. The contents of the aforementioned application are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a fuel cell system provided with a phosphoric acid fuel cell stack including a plurality of stacked power-generating cells each having a joined unit including an electrolyte impregnated with phosphoric acid and interposed between an anode electrode and a cathode electrode in which the joined unit is interposed between separators. The present invention also relates to a method for operating the fuel cell system.
2. Description of the Related Art
For example, the phosphoric acid fuel cell (PAFC) has a power-generating cell constructed such that a joined unit, which comprises an anode electrode and a cathode electrode principally composed of carbon respectively and provided opposingly on both sides of an electrolyte layer composed of a silicon carbide porous member (matrix) impregnated with concentrated phosphoric acid, is interposed between separators (bipolar plates). Usually, a predetermined number of the power-generating cells are stacked to be used as a fuel cell stack.
In such a fuel cell stack, a fuel gas such as a gas principally containing hydrogen (hereinafter referred to as xe2x80x9chydrogen-containing gasxe2x80x9d as well), which is supplied to the anode electrode, contains hydrogen which is ionized into ion on the catalyst electrode, and the ion is moved toward the cathode electrode via the electrolyte. The electron, which is generated during this process, is extracted for an external circuit, and the electron is utilized as DC electric energy. An oxygen-containing gas, for example, a gas principally containing oxygen (hereinafter referred to as xe2x80x9coxygen-containing gasxe2x80x9d as well) or air is supplied to the cathode electrode. Therefore, the hydrogen ion, the electron, and the oxygen are reacted with each other on the cathode electrode, and thus water is produced.
In general, in the case of the phosphoric acid fuel cell, the power generation is performed at a rated output (rated load) in a state in which the operating temperature is relatively high (about 120xc2x0 C. to 200xc2x0 C.). The product water, which is generated during the operation, is converted into steam which is discharged to the outside. However, if the phosphoric acid fuel cell is operated in a state of low load, the amount of self-heat generation of the phosphoric acid fuel cell is lower than the amount of heat release, resulting in the decrease in temperature of the phosphoric acid fuel cell. As a result, the product water, which has been evaporated at a high temperature, is pooled as liquid water in the electrolyte layer. A large amount of liquid water, which can not be kept in the electrolyte layer and the electrode, may be introduced into the reaction gas flow passage.
However, the phosphoric acid of the electrolyte layer has extremely high hydrophilicity. The phosphoric acid may flow out from the electrolyte layer and the electrode together with the liquid water introduced into the reaction gas flow passage. For this reason, the phosphoric acid is deficient in the electrolyte layer, the power generation performance of the power-generating cell is lowered, and the service life of the fuel cell itself is shortened.
In order to solve the problem of this type, an operating temperature control apparatus for a fuel cell is known, as disclosed, for example, in Japanese Laid-Open Patent Publication No. 64-27164. In this conventional technique, as shown in FIG. 5, a load 2 such as a motor is connected to output terminals (collecting electrodes) of a main cell body 1 of a fuel cell. A cooling system 3 for supplying cooling air into the main cell body 1 is arranged and connected to the main cell body 1. The cooling system 3 comprises an air-circulating tube passage 5 which includes a blower 4, an intake tube passage 5a which is branched from the air-circulating tube passage 5 and which is open to the atmospheric air via a variable damper 6, and a discharge tube passage 5b which is open to the atmospheric air.
A heater 7 for heating the cooling medium is arranged for the air-circulating tube passage 5. The electric power is supplied to the heater 7 from the fuel cell via a control unit 8. The control unit 8 controls the heater 7 and the opening degree of the variable damper 6 simultaneously based on an output signal of a temperature sensor 9 arranged in the main cell body 1.
In the arrangement as described above, the opening degree of the variable damper 6 is adjusted on the basis of the output signal of the temperature sensor 9 in a high load operation area in which the temperature of the main cell body 1 is high, and thus the operating temperature is maintained to be appropriate. On the other hand, when the temperature of the main cell body 1 is lowered as the load of the fuel cell is lowered, then the variable damper 6 is closed under the action of the control unit 8 based on the temperature detected by the temperature sensor 9, and the intake of new atmospheric air is stopped. Simultaneously, the control unit 8 operates the heater 7 to raise the temperature of the cooling air flowing through the cooling system 3. Accordingly, the amount of heat, which is required to increase the operating temperature of the main cell body 1, is supplied.
However, in the case of the conventional technique described above, the control reference, which is based on the output signal from the temperature sensor 9 arranged in the main cell body 1, is the proper operating temperature for the fuel cell (power-generating cell unit). Therefore, it is necessary that the control reference is set for each of the individual cell units which constitute the fuel cell stack. Further, the temperature distribution is made in the stacking direction as well as in each of the fuel cells in the main cell body 1. It is necessary to arrange a considerably large number of temperature sensors 9 in order to perform the control accurately. Therefore, the entire arrangement of the fuel cell stack is complicated.
Further, although a part of the temperature-adjusting medium circulates along the air-circulating tube passage 5 of the cooling system 3, the residual part of the temperature-adjusting medium is discharged from the discharge tube passage 5b. Therefore, the temperature-adjusting medium having high temperature is released to the outside of the cooling system 3, and the efficiency of increase in temperature of the main cell body 1 is lowered.
A principal object of the present invention is to provide a fuel cell system which makes it possible to efficiently maintain the operating temperature at a proper temperature with a simple arrangement and simple steps, and a method for operating the fuel cell system.
In the present invention, there are provided a medium-circulating passage for supplying, in a circulating manner, a temperature-adjusting medium into a phosphoric acid fuel cell stack, and first and second temperature sensors for detecting medium temperatures at a medium inlet and a medium outlet of the phosphoric acid fuel cell stack. Accordingly, it is possible to reliably detect, with only the first and second temperature sensors, the balance between the amount of self-heat generation and the amount of heat release of the entire phosphoric acid fuel cell stack, i.e., whether or not the operating temperature of the phosphoric acid fuel cell stack is within a proper operating temperature region. Thus, it is possible to easily simplify the structure.
Further, it is unnecessary to define the judgment reference value (control reference) for each of power-generating cells. It is possible to judge whether or not the entire phosphoric acid fuel cell stack is at the proper operating temperature by only detecting the medium temperatures at the medium inlet and the medium outlet of the phosphoric acid fuel cell stack by using only the first and second temperature sensors. Accordingly, the fuel cell system can be widely utilized in various applications.
Further, the medium-circulating passage is arranged with a pump for circulating the medium, a heater for heating the medium, a heat exchanger for cooling the medium, a bypass passage for bypassing the heat exchanger, and a medium flow passage-switching means for selectively supplying the temperature-adjusting medium to the heat exchanger and the bypass passage.
Accordingly, the temperature-adjusting medium is heated in the medium-circulating passage by the aid of the heater for heating the medium during the low load operation. The heated temperature-adjusting medium circulates through the phosphoric acid fuel cell stack. Thus, the temperature of the phosphoric acid fuel cell stack can be efficiently raised up to a proper operating temperature. During this process, the heated temperature-adjusting medium circulates through the bypass passage without releasing the heat at the heat exchanger. Thus, it is possible to efficiently raise the temperature of the phosphoric acid fuel cell stack.
As described above, the temperature-adjusting medium is selectively supplied to the bypass passage and the heat exchanger installed in the medium-circulating passage, and it circulates through the medium-circulating passage. It is possible to suppress the release of the heat of the temperature-adjusting medium to the minimum. Therefore, it is possible to suppress and minimize the electric power (electric power supplied to the heater for heating the medium) to be used in order to maintain the phosphoric acid fuel cell stack at the proper operating temperature. Further, it is possible to avoid any sudden decrease in temperature of the phosphoric acid fuel cell stack. Thus, it is possible to reliably perform the maintenance of the proper operating temperature even if the low load is used.
On the other hand, when the phosphoric acid fuel cell stack arrives at the proper operating temperature, the supply of the electric power to the heater for heating the medium is stopped. Subsequently, the temperature-adjusting medium, which circulates through the medium-circulating passage, is cooled as it passes through the heat exchanger under the action of the medium flow passage-switching means. Accordingly, it is possible to reliably maintain (cool) the phosphoric acid fuel cell stack to the proper operating temperature.
The above and other objects, features, and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which a preferred embodiment of the present invention is shown by way of illustrative example.